1. Field of the Invention
This invention relates to a potentially economically viable process for the destruction of small to large quantities of halogenated hydrocarbons, their homologous/analogues, and similar hazardous chemicals at ambient conditions using the superoxide ion in deep eutectic solvents.
2. Background of the Related Art
The handling, storage, and destruction of hazardous wastes (which includes polychlorinated biphenyls (PCBs), hexachlorohexane (HCB), and most chlorinated organics) is regulated by the Toxic Substances and Control Act (TSCA). Such wastes occur in the public and private sectors. The 1996 Integrated Data Base (IDB) report (DOE/RW Integrated database report 1995: U.S. Spent Nuclear Fuel and Radioactive Waste Inventories, Projections, and Characteristics. DOE/RW-0006, Rev. 12. December 1996) shows over 6.9 million kg of TSCA wastes in the Department of Energy (DOE) complex alone, and more is being generated. In 1995, an additional 543,000 kg of PCB waste was generated throughout the DOE complex, some of which is due to waste remediation and to interim storage of waste awaiting treatment. Although industry is making strides in reducing the usage of chlorinated solvents, a recent survey shows approximately 341,000 kg of trichlorethylene, perchloroethylene, trichloroacetylene, and other chlorinated solvents being used in part by dry cleaning, critical cleaning, paint stripping, and similar operations, Callahan and Green (Hazardous Solvent Source Reduction. 1995, McGraw-Hill: New York). Additional significant volumes of chlorinated hazardous wastes exist in landfills, brownfields, and other contaminated sites. Polyhalogenated aromatic hydrocarbons, which include materials such as PCBs and HCB, represent a major environmental problem. These materials contain components that are animal carcinogens and can cause birth defects, and their continual release into the ecosystem has a deleterious effect on animal life, Kalu and White (J. Electrochem. Soc.; 138: 3656 (1990)). Hexachlorobenzene, which is a byproduct of the poly(chloroethylene) solvent industry, is as environmentally persistent as PCBs and is a human carcinogen, Sugimoto et al. (Environ. Sci. Technol.; 22: 1182 (1988)).
Thermal incineration (oxidation) is the standard method of destroying chlorinated hydrocarbons, and is currently the only legal method of destruction of PCBs. Noyes reviews commercial and near commercial of hazardous waste incineration (Handbook of pollution control processes. 1991. Noyes Publications: Park Ridge, N.J.). Destructive oxidation is done in high-efficiency thermal incinerators or in cement kilns, but the potential for emission of dioxins makes EPA regulations on these operations very strict. Installing and operating direct thermal incineration plants incurs significant public resistance. Another high temperature destruction methods include injection of the waste into a moving bed of molten sodium carbonate salt. Supercritical water oxidation (SCWO) employs high temperature and pressure (>300° C., 20 MPa typically), at which state both oxygen and hydrocarbons become very soluble and destruction rates high, Savage (Chem. Rev.; 99: 603 (1999)) and Anitescu, and Tavlarides (Ind. Eng. Chem. Res.; 39: 583 (2000)). The severe process conditions require high-pressure vessels, and corrosion is a significant limitation. However, commercial installations of SCWO are appearing, and sufficient information for economic evaluation is being generated. Direct chemical oxidation is near ambient process conditions in which strong inorganic oxidants (e.g. sodium peroxydisulfate) in solution are used to destroy low concentration chlorinated waste (DOE/EM-0459 Direct Chemical Oxidation. Innovative Technology Summary Report, prepared for the U.S. Department of Energy, December 1998). Photochemical oxidation is also at the research stage, in this process, photocatalysts such as TiO2 are used to generate active hydroxyl or superoxide radicals in aqueous media, Jones and Watts (J. Env. Eng.: 974 (1997)). Biological destruction of chlorinated pollutants is receiving much attention, especially for in situ remediation; at present the kinetics are very slow, Jiro and Takayanagi (Tetrahedron; 34: 641 (1978)). Sawyer (Oxygen Chemistry, New York: Oxford University Press Inc., 1991) discussed in detail the reactions of O2•− with halogentaed hydrocarbons. Although the superoxide ion is a powerful nucleophile in aprotic solvents, it does not exhibit such reactivity in water, mainly because of its strong solvation and its rapid hydrolysis and disproportionation. The reactivity of O2•− with alkyl halides via nucleophilic substitution was first reported in 1970 (Kolarz and Rapak Makromolekulare Chemie; 185: 2511 (1984)) and (Rao and Perlin, Canadian Journal of Chemistry, 59: 333 (1981)). Both 1,1,1-Trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and methoxychlor are rapidly deprotonated by HO− in aprotic solvents with subsequent elimination of Cl− to form dehydrochlorination products; that is, DDT forms 4,4′-dichlorodiphenyldichloroethane1,1-dichloro-chlorobenzene (DDE). The same products are formed in their reaction with O2•−.
Merritt and Sawyer (J. Org. Chem.; 35: 2175 (1970)) studied the kinetics of the reactions of tetraethylammonium superoxide in dimethyl sulfoxide. They showed that stable solutions of tetraethylammonium superoxide could be prepared in aprotic solvents by electrochemical generation.
Sugimoto et al. (Sci. Technol.; 22: 1182 (1988)) reported that polyhalogenated aromatic hydrocarbons (e.g., PCB's and hexachlorobenzene) are rapidly degraded by superoxide ion in dimethylformamide to carbonate and halide ions. The efficient destruction of such materials accomplished via the in situ electrolytic reduction of dioxygen to generate the superoxide ion, which reacts with poly aromatics by nucleophilic substitution. The reaction stoichiometries have been determined by cyclic voltammetric measurements, and the reactant/product profiles have been assayed by capillary gas chromatography and potentiometric titrations.
Analogous complete destruction by superoxide ion occurs for PCB's that contain three or more chlorine atoms per aromatic ring. Another means to the dehalogenation of haloaromatic hydrocarbons is their electrolytic reduction to the parent hydrocarbon in oxygen-free dimethylformamide solutions. Electrochemical studies confirmed that all PCB's can be dehalogenated via anaerobic electrolysis.
The nucleophilic addition of superoxide must take place in an anhydrous environment. The electro-generation of O2•− from dissolved air represents a much more practical approach for an effective system to destroy PCB's and other halogenated aromatic hydrocarbons. The authors also suggested a reactor for the destruction process. They indicated that a reasonable electrolytic reactor to process such wastes would involve graphite cathodes and anodes with Me4NCl as the supporting electrolyte in dimethylformamide and the cathode compartment saturated with air or pure O2 at 1 atm. PCB's would be introduced into the cathode compartment and degraded by the reductive activation of the dioxygen. The anode reaction for this reactor involves the oxidation of the chloride ion produced in the cathode compartment. Thus, the overall electrolysis reaction is the equivalent of an electrostimulated combustion:C6Cl6+6O2→6CO2+3Cl2  (1)
When Arochlor 1268 (a commercial PCB fraction that contains a mixture of Cl7, Cl8, Cl9, and Cl10 polychlorobiphenyls) is combined with excess O2•−, the entire mixture is degraded. Samples taken during the course of the reaction confirm that the most heavily chlorinated members react first, the initial nucleophilic addition is the rate-determining step, and all components are completely dehalogenated. Tests with other PCB mixtures establish that those components with three or more chlorine atoms per phenyl ring are completely degraded by O2•− within several hours.
Kalu and White (J. Electrochem. Soc.; 138: 3656 (1990)) studied the effects of current, electrolyte flow, and aprotic media on the extent of degradation of HCB by superoxide ion the electrochemically generated superoxide ion in a flow cell system equipped with a gas fed, porous electrode. The authors found that that the degradation efficiency reported by Sugimoto et al., almost 100% for a batch system was not duplicated for a once-through flow reactor. The disagreement between the results of this work and the batch results of Sugimoto et al. indicated the complexity introduced into the system performance due to the flow effects. Theoretically, the closer the flow approaches zero, the more likely the 100% degradation efficiency will be approached. Their preliminary work showed that in-situ generated superoxide ion in once-through flow reactor, equipped with porous electrode, could be used for the partial degradation of hexachlorobenzene. The anode reaction, thought to be chlorine oxidation, did not interfere with the homogenous chemical reaction between the superoxide ion and the poly-chlorinated substrate. This is because no observation was made of chlorinated products other than C6Cl6. The authors found that the flow rates or residence time of the electrolyte in the reactor and the applied current density affected the level of substrate destruction achievable. They found that the type of solvent in use also influenced the degradation efficiency. Their experimental results demonstrated that, under the same operating conditions, the percentage degradation achievable was highest in DMF/DMSO solvent.
Although they are proven highly toxic and carcinogenic, chlorinated hydrocarbons (CHC's) are still widely used in the manufacturing of many chemical compounds, such as herbicides, fungicides and pharmaceuticals. CHC's are also applied in dry cleaning processes, in degreasing operations and as organic solvents.
The current method to remove CHC's is thermal incineration at temperatures higher than 1300° C. These high temperatures are required to avoid the formation of dioxins and PCB's. Because of the high incineration temperatures and consequently, the high costs, scientists are forced to look for other but cheaper alternatives that are not harmful to the environment.
A first alternative process is the catalytic oxidation of CHC's at temperatures between 300° C. and 550° C. over supported noble metal catalysts (e.g. Pt, Pd and Au). The essential drawback here is the deactivation of the catalyst by the decomposition products including Cl2 and HCl. Another disadvantage is the formation of volatile oxychlorides, which can condense and block the installation in the colder parts of the reactor. In contrast, it has been stated that supported transition metal oxide catalysts are resistant to these kinds of deactivation. Among these types of oxides Cr2O3 seems to be the most promising catalyst for the total oxidation of CHC's. Frequently used supports are Al2O3, TiO2 and SiO2. Other classes of materials are zeolites (e.g. H—Y and H-ZSM-5 zeolites), perovskites (e.g. LaCoO3 and LaMnO3) and pillared clays.
A second alternative for incineration is hydro-dechlorination in which a CHC is transformed in the presence of hydrogen into the corresponding alkane and HCl. Commonly used catalysts are Pd and Pt on various supports. Ni/SiO2 catalysts also seem to possess a high activity. Although this technique has economic and environmental advantages, including the re-use of the reaction products and the elimination of hazardous by-products (e.g. Cl2 and COCl2), it is not used very often. The main reason is the very fast deactivation of the catalyst material. This deactivation is probably due to the interaction between HCl and the catalyst and to coke formation caused by oligomers formed on the acid sites of the catalyst. A third alternative for incineration was provided by Weckhuysen et. al. (J. Phys. Chem. B,; 102: 3773-3778 (1998)). They have studied the destructive adsorption of carbon tetrachloride on alkaline earth metal oxides, more specifically BaO, SrO, CaO and MgO. They concluded that alkaline earth metal oxides are active materials for the destructive adsorption of carbon tetrachloride in the absence of O2. The destruction activity parallels the basicity of the alkaline earth metal oxide. Carbon tetrachloride destruction was accompanied by the formation of chlorides (BaCl2; SrCl2, CaCl2 and MgCl2 in the case of BaO, SrO, CaO and MgO, respectively). They observed that the resulting barium chloride is recyclable by dissolving the solid in water, followed by precipitation and heating in oxygen. The biggest disadvantage of this technique; however, is that it is a stoichiometric and not a catalytic process. This means that, once the metal oxide is converted to the corresponding chloride, the activity of the system falls back to almost undetectable destruction levels.
U.S. Pat. No. 4,561,969 provides a process for the removal of the halogen moiety from halogenated hydrocarbon feedstock. The homogenous process described in this patent depends on the use of sulfuric acid and a lanthanide oxide, the latter being required to break the chlorine ion from the hydrocarbon in order to form a chlorosulfonic acid. The oxides are regenerated by bubbling O2 through the depleted H2SO4 solution.
U.S. Pat. No. 5,358,65 disclosed compositions suitable for use in degrading and detoxifying polyhalogenated organic compounds comprising an aprotic solvent having dissolved therein (a) an effective amount of hydrogen donor, (b) an effective amount of a compound which produces hydroxide ion or alkoxide ion, and (c) dioxygen, are shown. These reagent compositions may be used to produce superoxide ion in situ for use in a variety of industrial applications to degrade halogenated hydrocarbons, e.g., PCBs. The generation of superoxide ion may be catalyzed by the presence of anthraquinone and derivatives thereof. Reagent compositions containing (a) an effective amount of hydrogen donor, e.g., hydroxylamine, (b) an effective amount of a compound which produces hydroxide ion or alkoxide ion and (c) dioxygen were also disclosed. In preferred methods the dioxygen is bubbled through the solutions to continuously form superoxide ion.
U.S. Pat. No. 6,222,089 disclosed a process for destroying aromatic compounds in an exhaust gas containing also sulfur dioxide, nitrogen oxides and dust, first the dust is removed at a temperature of between room temperature and 150° C. to a residual quantity of not more than 50 mg/m3, the sulfur dioxide is removed to a residual quantity of not more than 1000 mg/m3, the nitrogen oxides are removed to a residual quantity of not more than 300 mg/m3 and the exhaust gas is then subjected to electron irradiation having an energy of 3 to 10 kWh/1000 m3 exhaust gas.
U.S. Pat. No. 6,127,592 disclosed an in situ hydrous pyrolysis/oxidation process that is useful for in situ degradation of hydrocarbon water and soil contaminants. Fuel hydrocarbons, chlorinated hydrocarbons, polycyclic aromatic hydrocarbons, petroleum distillates and other organic contaminants present in the soil and water are degraded by the process involving hydrous pyrolysis/oxidation into non-toxic products of the degradation. The process uses heat which distributed through soils and water, optionally combined with oxygen and/or hydrocarbon degradation catalysts, and is particularly useful for remediation of solvent, fuel or other industrially contaminated sites.
U.S. Pat. No. 5,986,147 disclosed a method and apparatus for decontamination of soil from contaminates such as polychlorinated biphenyls. The method involves (a) admixing a short chain alcohol and a ethoxylated alcohol to provide a solvent composition, (b) admixing the solvent composition with the contaminated soil to extract the contaminant from the soil into the solvent to form contaminated solvent, (c) separating the soil from the contaminated solvent, (d) washing of the decontaminated soil with water to remove residual solvent from the soil, and (e) distillation of the contaminated solvent to separate the reusable short chain alcohol as a light fraction from a heavy fraction containing the ethoxylated alcohol and the contaminant. The apparatus includes (a) a mixer for mixing the soil and solvent, (b) a centrifuge for separating fines from decanted contaminated solvent, (c) a distillation unit for separating the solvent into a reusable light fraction and a waste heavy fraction, and (d) a water reclamation unit for removing residual solvent from the wash water. The method and apparatus are usable for reclaiming contaminated soils.
U.S. Pat. No. 5,569,809 disclosed a method for detecting and destroying various chlorinated hydrocarbons and unsaturated hydrocarbons in the environment using electrochemical techniques. By concentrating the chlorinated hydrocarbons or unsaturated hydrocarbons in a solvent by liquid-solid or liquid-liquid extraction, and then passing current through the solvent extractant containing the chlorinated hydrocarbons or unsaturated hydrocarbons electrochemically removes chlorine and oxidizes unsaturated hydrocarbons. The potential is measured to correlate to the concentration of the chlorinated hydrocarbon species.
U.S. Pat. No. 5,637,198 disclosed an invention drawn to a volatile organic compound environment effective reduction apparatus comprising a reactor-efficient coronal discharge zone, and at least one pair of high-dielectric coated electrodes. In particular embodiments halogenated volatile organic compounds are reduced and the reaction chamber is chlorine and fluorine resistant.
U.S. Pat. No. 5,564,351 disclosed a process and equipment for pyrolytic decomposition of chlorinated compounds in a molten metal bath in the absence of air to elemental products with recovery of the carbon and chlorine is disclosed.
U.S. Pat. No. 5,393,428 disclosed a process for treating waste water containing chlorinated organic substances, particularly waste water from production of epichlorohydrin which contains more than 10 mg of adsorbable organic halogen compounds per liter, comprising charging the waste water at a pH of 10 to 14 into a reactor and maintaining a temperature of at least 75° C., a pressure of at least 1 bar, and a residence time of at least 0.5 hours in said reactor, thereby partially dechlorinating and/or dehydrochlorinating chlorinated organic compounds contained in the waste water, and thereafter subjecting the waste water to further dechlorination and/or dehydrochlorination treatment in the presence of a hydrogen-containing gas, a hydrogen-releasing compound and/or a catalytically active material; and optionally subjecting the waste water to a biological treatment with the use of microorganisms; and apparatus for carrying out the foregoing process.
U.S. Pat. No. 5,024,949 disclosed a bacterium of the genus Pseudomonas which utilizes a branched chain alkyl-substituted aromatic hydrocarbon as its sole carbon and energy source and which capable of substantially complete degradation of trichlorethylene, and methods utilizing the bacterium for the detoxification of trichlorethylene-contaminated material.
U.S. Pat. No. 4,541,907 disclosed a process for decomposing chlorinated hydrocarbon compounds wherein the compounds are heated to an elevated temperature of about 800 to 1200° C. and reacted with alumina and a reactive carbon source. A preferred operating temperature is about 800° C. to 900° C. and preferred carbon sources are activated carbon and coked alumina. The process is particularly useful for disposing of chlorinated organic compounds that are generated in the production of metal chlorides by chlorination of a metal oxide in the presence of carbon.
U.S. Pat. No. 4,423,024 disclosed a method of converting saturated aliphatic chlorinated hydrocarbons to carbon dioxide and hydrogen chloride which comprises contacting a pre-heated mixture of the hydrocarbon, oxygen and, where necessary, water with a molecular sieve catalyst at temperatures of from about 180 to about 400° C. Simple chlorinated hydrocarbons can thus be completely converted to HCl and CO2 at low temperatures without the production of undesired by-products.
U.S. Pat. No. 5,447,639 disclosed a method for the remediation of aqueous solution contaminated with chlorinated aliphatic hydrocarbons. The chlorinated hydrocarbons are reduced to ethane, ethene, and chloride ion. In-situ reactions can take place in a ditch or screened well. Ex-situ reactions can take place in columns packed with ferrous sulfide.
U.S. Pat. No. 5,994,604 disclosed a method and apparatus for decomposing halogenated hydrocarbons are provided. The halogenated hydrocarbon is mixed with solvating agents and maintained in a predetermined atmosphere and at a predetermined temperature. The mixture is contacted with recyclable reactive material for chemically reacting with the recyclable material to create dehalogenated hydrocarbons and halogenated inorganic compounds. A feature of the invention is that the process enables low temperature destruction of halogenated hydrocarbons.
U.S. Pat. No. 6,096,283 disclosed an integrated system for destruction of organic waste comprises a hydrolysis step at moderate temperature and pressure, followed by direct chemical oxidation using peroxydisulfate. This system can be used to quantitatively destroy volatile or water-insoluble halogenated organic solvents, contaminated soils and sludges, and the organic component of mixed waste. The hydrolysis step results in a substantially single phase of less volatile, more water soluble hydrolysis products, thus enabling the oxidation step to proceed rapidly and with minimal loss of organic substrate in the off-gas.
U.S. Pat. No. 5,575,927 disclosed a method for the reduction halogenated hydrocarbons wherein iron and ferrous sulfide in specific amounts are combined under aerobic or anaerobic conditions to reduce halogenated hydrocarbons in aqueous compositions to non-toxic by-products at near neutral pH. The combination of iron metal and ferrous sulfide in relative amounts gives the unexpected result of a faster reduction rate using iron or ferrous sulfide alone and a controlled pH.
A new class of compounds, ionic liquids has emerged in the last ten years that may become a key ally in meeting the twin challenges of efficient and environmentally benign chemical processing. They have the potential to revolutionize the way we think of and use solvents. The reason is, they act like good organic solvents, dissolving both polar and nonpolar species. In many cases, they have been found to perform better than commonly used solvents. In addition, ionic liquids are non-flammable and non-volatile. The wide and readily accessible range of ionic liquids with corresponding variation in physical properties offers the opportunity to design an ionic liquid solvent system optimized for a particular process.
A key feature of ionic liquids is that their physical and chemical properties can be tailored by judicious selection of cation, anion, and substituents. For example, a choice of anions such as halide (Cl−, Br−, I−) nitrate (NO3−), acetate (CH3CO2−), trifluoroacetate (CF3CO2−), triflate (CF3SO3−) and bis(trifluoromethylsulfonyl) imide (CF3SO2)2N−) can cause dramatic changes in the properties of ionic liquids. The water solubility of the ionic liquid can be controlled by the nature of the alkyl substituent on the cation. Increasing the length of the alkyl chain tends to decrease water solubility by increasing the hydrophobicity of the cation.
ILs, however, are quite difficult to make, very expensive, and their toxicity has not yet been ascertained. Accordingly, none of these compounds has been registered and this currently limits their wide-scale use. In addition, only a small number of ILs is being produced in commercial quantities.
A deep eutectic solvent (DES) is a type of ionic solvent with special properties composed of a mixture which forms a eutectic with a melting point much lower than either of the individual components. The first generation eutectic solvents were based on mixtures of quaternary ammonium salts with hydrogen donors such as amines and carboxylic acids. The deep eutectic phenomenon was first described in 2003 for a 1 to 2 by mole mixture of choline chloride (2-hydroxyethyl-trimethylammonium chloride) and urea. Choline chloride has a melting point of 302° C. and that of urea is 133° C. The eutectic mixture however melts as low as 12° C.
This DES is able to dissolve many metal salts like lithium chloride (solubility 2.5 mol/L) and copper(II) oxide (solubility 0.12 mol/L). In this capacity, these solvents could be applied in metal cleaning for electroplating. Because the solvent is conductive, it also has a potential application in electropolishing. Organic compounds such as benzoic acid (solubility 0.82 mol/L) also have great solubility and this even includes cellulose (filtration paper). Compared to ordinary solvents, eutectic solvents also have a very low VOC and are non-flammable. Other deep eutectic solvents of choline chloride are formed with malonic acid at 0° C., phenol at −40° C. and glycerol at −35° C.
Compared to ionic liquids that share many charactistics but are ionic compounds and not ionic mixtures, deep eutectic solvents are cheaper to make, much less toxic and sometimes biodegradable.
WO 2002 026381 disclosed an invention related to ionic compounds and methods for their preparation. In particular, the invention relates to ionic compounds comprising hydrated metal salts, which are liquid at low temperatures, generally below about 100° C.
WO 02/26701 A2 disclosed a method for the synthesis of DES compounds with a freezing point of up to 100° C. by the reaction of one amine salt (I), such as choline chloride with an organic compound (II) capable of forming a hydrogen bond with the anion of the amine salt, such as urea, wherein the molar ratio of I to II is from 1:1.5 to 1:2.5. The DES compounds are useful as solvents, and electrolytes for example in electroplating, electrowinning, electropolishing, and as catalysts.
WO 00/56700 disclosed a method for the synthesis of DES having a melting point of no more than 60° C., formed by the reaction of a quaternary ammonium compound or a mixture of two or more thereof; with a halide of zinc, tin or iron, or a mixture of two or more thereof.
We were the first to show that a stable superoxide ion can be generated in ILs [AlNashef et al. Ph. D. Dissertation, 2004]. We also showed that hexachlorobenzene could be destroyed by the reaction of the superoxide ion generated in selected ILs.
From what was mentioned above, it is clear that there is a need for a viable method for the destruction of halogenated hydrocarbons that is inexpensive, occurs at low temperature, and most importantly, benign.